Field
This invention pertains generally to circuit interrupters and, more particularly, to circuit interrupters including a trip unit and a power supply. The invention also relates to a power supply start-up circuit for a circuit interrupter trip unit.
Background Information
Circuit breakers and circuit breaker trip units are well known in the art. See, for example, U.S. Pat. Nos. 5,910,760; 6,144,271; and 6,850,135.
Circuit breaker trip units require power for both energizing and tripping the trip actuator of the trip unit and energizing the signal processing circuitry of the trip unit. Power for trip units is typically provided by an iron core current transformer (CT), which may or may not also provide primary current indication (i.e., the current indication used for monitoring for overcurrent conditions). Generally, this CT is regulated to provide a relatively large output voltage to a capacitor which stores energy that is needed to energize and trip the trip actuator of the trip unit. Because this CT is capable of supplying only a certain amount of power, a relatively efficient switching power supply is preferred over a relatively less efficient linear regulator to convert the capacitor voltage to a relatively smaller voltage supply (at a relatively higher current) for the signal processing circuitry of the trip unit. Preferably, current-powered trip units run at the lowest possible CT primary current, or equivalently at the lowest possible load current flowing through the circuit breaker. This is desirable for both display/metering purposes and for protection purposes.
A conventional switching regulator integrated circuit may be electrically connected to receive the capacitor voltage. However, this configuration does not provide the lowest possible current power-up of the trip unit.
Given a fixed load requirement and being approximately constant power devices, all switching regulators draw more input current from their input power supply at a relatively lower input voltage than at a relatively higher input voltage. Therefore, for a current-limited source, such as a CT secondary when operated at relatively low primary current, the switching regulator starting at its minimum operating voltage will require greater input supply current than a regulator starting at a higher voltage. At relatively low primary current, the current output of CT secondary is limited by the CT primary current divided by the number of secondary turns even though itsvoltage output in an unloaded state can be quite high. In other words the CT is a current source providing large voltage and limited current. The switching regulator as a load requires decreasing current with increasing input voltage.There is a need to balance these two facts.
Accordingly, there is room for improvement in circuit interrupters and in power supplies for trip units.